Writing my first blender Addon and I am confused on these string keywords
The example "strings" that you show belong to two different categories, frame_start and frame_end are property names while render.render is an operator name. The properties will display as a number that you can edit while the operator will show as a button that will perform an action when clicked.

The strings that you use need to match an existing property or operator for them to work, an operator is identified by it's bl_idname property. You can specify a property or operator name and have a different string shown by passing a text paramater.

row.prop(scene, 'frame_start', text='adjust this here')
row.operator('render.render', text='This is my render button')


To create extra properties that are only used by your addon, you can use custom properties, which you should create in your addons register() function.

class MyPanel(bpy.types.Panel):
    ...
    def draw(self, context):
        row = self.layout.row()
        row.prop(context.scene, 'my_prop')

def register():
    bpy.types.Scene.my_prop = bpy.props.IntProperty()
    bpy.utils.register_class(MyPanel)


For operators, you can make your own and then use the bl_idname that you set in your operator class to show a button for it. If no text paramater is passed, an operators bl_label property is shown on the button.